1. Field
The present disclosure relates generally to vehicles and, in particular, to spacecraft. Still more particularly, the present disclosure relates to a method and apparatus for navigating spacecraft using global positioning system signals.
2. Background
Spacecraft navigate through space using information from the ground. Oftentimes, a spacecraft receives commands from the ground or commands from an operator inside the spacecraft. Commands may be received by the spacecraft from ground stations to provide a flight path for the spacecraft. Based on these commands, thrusters and other devices may be operated to place the spacecraft in a desired location or orbit above the earth.
This type of navigation for spacecraft involves having ground stations on the earth. These ground stations transmit commands and other information to a spacecraft to control the movement of the spacecraft. If insufficient ground stations are present in different locations on the earth, a spacecraft may be unable to receive commands during some periods of time when passing over portions of the earth where ground stations are absent. These periods of time occur when the spacecraft is unable to communicate with the ground stations. As a result, the spacecraft may be lost or unable to maneuver at times because of the loss of ground control.
Ground control systems typically have numerous ground stations in various locations on the earth to provide as much coverage as possible for sending commands to spacecraft. Ideally, the ground stations provide continuous coverage for the spacecraft. This situation, however, is not always possible. These types of systems may be more expensive than desired because of the number of ground stations used to communicate with the spacecraft when the spacecraft is in different positions over the earth.
Further, in some cases, placing a ground station in a desired location may be difficult. Obtaining permission from a particular country or other entity may be difficult in some cases.
Therefore, it would be desirable to have a method and apparatus that takes into account at least some of the issues discussed above as well as possibly other issues.